Monoamines as mediators of the antinociceptive effect of baclofen.

The antinociceptive activity of baclofen was evaluated in rats using the tail flick and hot plate tests. When baclofen was administered intraperitoneally, pretreatment with depletors of monoamines and catecholamines (reserpine, alpha-MPT) produced a marked increase in antinociceptive activity in both tests. Blockade of noradrenergic and dopaminergic receptors with phentolamine, chlorpromazine and haloperidol produced similar increases. These agents were without intrinsic effect in these tests except for alpha-MPT which increased the tail flick latency slightly. Alterations in serotonergic function did not consistently alter the action of baclofen. When baclofen was administered intrathecally, pretreatment with reserpine and alpha-MPT produced minimal alterations in antinociceptive activity. It is likely that interactions between baclofen and monoamines occur at supraspinal sites.